Multiple Access Variable Code
Code Division Multiple Access is a well known multiplexing method. A wireless medium can be shared by multiple users by using different pseudo-random code sequencers for different users. The pseudo-random code sequencers encode received data streams by codes that are ideally orthogonal to each other.
A CDMA modulator includes an encoder that converts an input data stream into an encoded data stream that is transmitted over a wireless medium. The relationship between the size of a data stream, bit rate of the data stream or a bandwidth consumed by the data stream, before being CDMA encoded and between the size a CDMA encoded data stream, bit rate of the encoded data stream or a bandwidth consumed by the encoded data stream, is referred to as a CDMA encoding factor.
The following U.S. patents describe various methods and systems for CDMA encoding: U.S. Pat. No. 7,035,201 of Fu et al., titled “Programmable transceiver structure of multi-rate OFDM-CDMA for wireless multimedia communications”; U.S. Pat. No. 6,421,336 of Proctor, Jr., et al, titled “Variable rate orthogonally coded reverse link structure”; and U.S. Pat. No. 7,088,700 of Lee et al., titled “Apparatus and method for coding/decoding TFCI bits in an asynchronous CDMA communication system”.
Rate-Shaping Techniques
In many media generation and distribution systems multiple video streams are conveyed over a shared wired channel. The aggregate bandwidth of these video streams should not exceed the available bandwidth of the shared wired channel.
Various lossy and lossless techniques are implemented to adapt the aggregate bandwidth of the programs of a Transport Stream to the available bandwidth of a channel. U.S. Pat. Nos. 6,038,256 and 6,192,083 of Linzer et al, U.S. Pat. Nos. 5,862,140 and 5,956,088 of Shen et al and U.S. Pat. No. 5,877,812 of Krause et al, describe some of these prior art methods. Lossless techniques, such as statistical multiplexing, do not require further compressing of media pictures. Lossless techniques also include delaying or advancing a transmission of transport packets. Lossy techniques involve additional compression (also referred to as rate shaping), and are usually implemented whenever the appliance of lossless techniques is not feasible or does not provide sufficient results. The further compression usually results in visual quality degradation.
Typically, the rate-shaping is responsive to the bandwidth of the shared wired channel and ignored other limitations that can be imposed on the video streams. For example, the rate-shaping ignores any affect that CDMA encoding will have upon the media stream. Accordingly, two video streams of the same priority (same quality of service level) can be subjected to the same rate-shaping scheme but can be CDMA encoded and then transmitted at a totally different bit rate over a wireless medium, thus allocating more wireless medium resources to one media stream. Yet for another example, the CDMA code length can be dynamically altered to compensate for changes in the wireless link (for example—a certain wireless channel becomes noisier) but the rate-shaper is indifferent to these alterations.
Media Over Wireless Channels
As bandwidth capabilities increase with wireless technologies, users will want real-time and non-real-time video available for mobile download. This can be achieved with either the various mobile technologies of EV-DO with CDMA or with HSDPA (High Speed Downlink Packet Access) with GSM or alternatively the evolving technologies of WiMAX/802.16, Flash-OFDM/802.20 or future 4G standards. However, the wireless channel will remain a bandwidth limited channel and as the wideband features become more popular, there will be an increasing pressure to use the bandwidth efficiently and effectively. The higher the number of video/voice/music channels that can be compressed into a limited channel bandwidth while maintaining a certain minimum quality of media, the more revenue a carrier will be able to generate from said channel.
Wireless channels differ from wired channels in various aspects. On one hand, wired channels are: (i) static with respect to bandwidth capacity, (ii) have a fixed modulations scheme (for example QAM64 or QAM256), (iii) are relatively wide (some can carry approximately 30 Mbps and 40 Mbps respectively), (iv) the Per-user or Per-CPE capacity is also static, thus each user experiences virtually the same physical plant characteristics and has the ability to access the same portion (virtually 100%) of the channel.
On the other hand, wireless channels are characterized by: (i) rapid channel state alterations (including rapid channel capacity changes), (ii) the wireless channel between one user and a base station differs from the wireless channel, (iii) the wireless channels differ on a CPE-basis, and the like.
There is a need to provide efficient rate shaping and encoding systems.